The Mercer Mess still was messing with drivers Monday during the first morning commute with Mercer Street as a two-way boulevard.

Mercer opened to two-way traffic Sunday night after more than 40 years as a one-way arterial through South Lake Union. Many drivers weren't quite sure how to react upon taking the Mercer exits from Interstate 5.

Ongoing closures created more confusion. Southbound Fairview Avenue is closed for construction, so drivers don't have the usual outlet for heading south toward downtown.

And Valley Street, where drivers used to go to head west, now is cut off at Boren Avenue.

"We're seeing a lot of the old behvaiors with people trying to make a right turn onto Fairview or trying to make a left turn on Valley," said Rick Sheridan, a Seattle Department of Transportation spokesman. "We do expect the adjustment period to continue for several days to several weeks."

The mess generated heavier-than-usual backups on I-5, where congestion lasted well beyond 10 a.m. -- much later than usual, the state Department of Transportation reports.

"When we have something new, it usually takes about two weeks to get acclimated to it," WSDOT spokeswoman Kris Olsen said. "So we'll probably see this for a little bit."

A trip on southbound I-5 from Lynnwood to Seattle took about 40 minutes during the 9 a.m. rush hour. It normally averages about 18 minutes, according to WSDOT.

The peak travel time on northbound I-5 from Seatac to Seattle was 39 minutes. It normally averages about 26 minutes.

A disabled vehicle on the northbound off-ramp to Mercer between 7:40 a.m. and 8:30 a.m. also added to the congestion.

Police officers were at the intersection directing traffic. SDOT sent out alerts through media the last few weeks about the conversion and ongoing closures as the $164 million project continues.

SDOT has installed extra electronic message signs, including one at the Mercer off-ramp, to remind drivers of the changes and closures, said Sheridan, the SDOT spokesman.

Some 80,000 vehicles travel through the corridor each day. The old configuration was a source of daily bottlenecks, and SDOT expects that once drivers are accustomed to the new pattern, traffic will improve.

Westbound trips from I-5 to Seattle Center will improve by about four minutes during the morning commute. But trips in the same direction will be a minute slower in the evening rush hour, according to SDOT's traffic studies. Eastbound drivers probably won't notice any change -- no slower or faster, according to the city. But eastbound trips during the evening rush hour will be about three minutes quicker.

With Fairview Avenue closed, Westlake Avenue is first opportunity for drivers to turn left off of Mercer Street upon exiting the freeway.

The next big step in the $164 million project is to transform Valley Street from a busy arterial to a greener and more pleasant lakeside road with one lane in each direction and bike lanes. Eastbound traffic will be blocked at Terry Avenue, which will become an access point for traffic to Lake Union Park and businesses in Chandler's Cove.